1. Field of the Invention
The present invention relates to fuel cell systems, related operating methods and current measuring devices for use in fuel cell systems and, more particularly, to a fuel cell system, related method and current measuring device for use in the fuel cell system.
2. Description of the Related Art
Considerable research and development work has heretofore been undertaken in the past to provide fuel cell systems, employing fuel cells, serving as electric power generators for moving objects, such as vehicles, ships, portable electric power generators and electric power generators for domestic use.
In the state of the art fuel cell systems incorporating fuel cells that generate electric power through electrochemical reaction between hydrogen and oxygen, various defects take place during operations of the fuel cells. For instance, if the fuel cell encounters a shortage of moisture content, then, an electrolyte membrane is dried, causing a drop in power output. On the contrary, if liquid droplets collect in excess inside the fuel cell, electrodes of the fuel cell are covered with water to disturb gases to permeate, resulting in a drop in power output. Accordingly, a need arises in diagnosing a drying characteristic of the electrolyte membrane and a wetting characteristic of the electrode for appropriately controlling a water collecting condition and a wet condition inside the fuel cell. Also, another issue arises in a shortage of reaction gases to be supplied to the fuel cell with a resultant drop in power output. Thus, there is a need for properly diagnosing the occurrence of shortage of reaction gases to appropriately control the flow rates of reaction gases.
To overcome the above issues, various proposals have been made to provide technologies of detecting the occurrence of a drop in a cell voltage to diagnose defective conditions for thereby discriminating an operating condition of the fuel cell.
Japanese Patent Provisional Publication No. 9-259913 discloses technology of diagnosing a shortage of reaction gases depending upon an electric current distribution pattern of the fuel cell for controlling the flow rates of reaction gases or load current to avoid damage to a fuel cell.
With such technology of diagnosing the defective condition of the fuel cell depending upon the drop in cell voltage, since the cell voltage drops for any of reasons caused by the electrolyte membrane being dried, reaction disturbance as a result of water collected in excess inside the fuel cell and the shortage of reaction gases being supplied, a difficulty is encountered in specifying individual factors for the drop in power output and, therefore, an issue arises in a difficulty of appropriately controlling the fuel cell depending on the individual factors for the drop in power output.
In the meanwhile, with the above diagnosing technology, diagnosis has been done based on only the shortage of reaction gases and, so, it becomes impossible to properly diagnose other specific factors for the drop in power output. Therefore, it is hard to distinguish the occurrence of excess collected water and the occurrence of a dried status of the electrolyte membrane except for the flow rates of reaction gases to be supplied to the fuel cell. Consequently, serious issues occur in properly controlling the fuel cell on the individual factors that could have caused the drop in power output.
Further, during warmup operation immediately after startup of the fuel cell, pressures of fuel gas and air to be supplied to the fuel cell are insufficient and a reaction temperature of a fuel cell body is not adequately raised. Thus, an electric power generating capacity of the fuel cell does not reach a rated capacity of generating electric power, and the fuel cell remains in a low capacity of generating electric power. Under such a condition, if attempts are made to use electric power from the fuel cell at a rate exceeding the rated capacity of available electric power, a rapid drop occurs in cell voltage, causing deterioration in operating performance of the fuel cell.
U.S. Pat. No. 5,646,852 (issued to Lorenz et al) discloses a fuel cell system wherein an upper limit of electric power to be generated is restricted depending on a temperature of a fuel cell to preclude a power demand from being commanded at a rate greater than available power output.
With such an operating method, since the fuel cell takes the form of a structure wherein power output is limited depending upon the temperature of the fuel cell, the fuel cell encounters probabilities wherein during operation of the fuel cell at a low temperature, available power output to be generated by the fuel cell is restricted to a lower level than that available at normal operation and, hence, heat buildup is also restricted in the fuel cell, causing an increase in warmup time of the fuel cell. In addition, in cases where power output of the fuel cell is controlled depending on the temperature of the fuel cell, probabilities occur in which power output is controlled depending on the temperatures to an extent more than necessary.
Japanese Patent Provisional Publication No. 2002-343397 discloses a fuel cell system that is arranged to execute control to vary the flow rates and pressures of reaction gases in accordance with irregularities in output voltages of unit cells forming a fuel cell.
With such a control method, when detecting irregularities in cell voltages, an overall voltage of the unit cells is monitored. Under such a condition, if a drop occurs in a voltage of a defective unit cell to cause irregularities in cell voltages, the defective unit cell entirely undergoes an abnormal condition.
For this reason, even when attempting to recover the operation of the fuel cell subsequent to the occurrence of irregularities in cell voltages, a recovery time is prolonged or no recovery can be made. If no recovery of the fuel cell can be achieved, there occurs an issue in that a fuel cell powered vehicle comes to a halt on a road.
Furthermore, if water remains inside the fuel cell during stop thereof, water inside the fuel cell freezes under low temperature environments. When starting up the fuel cell under such low temperature circumstances, due to clogging occurring in reaction gas flow passages or the occurrence of disturbances in delivering reaction gases (hydrogen and air) to the electrolyte membrane, no electrochemical reaction takes place in the presence of fuel gas being supplied, causing a difficulty in starting up the fuel cell.
To address such an issue, Japanese Patent Provisional Publication No. 2002-343397 proposes a fuel cell system in which during stop of a fuel cell, air is supplied to the fuel cell for a given time interval to allow a stream of air to achieve air purge operation to remove moisture content from the fuel cell.
With such a fuel cell system, since the time interval for which purge air is supplied is preliminarily fixed, the air purge operation is carried out for a certain time interval regardless of moisture content inside the fuel cell. For this reason, if less moisture content exists in the fuel cell, probabilities occur where the air purge operation is carried for longer than required. Under such a situation, the fuel cell has a shortage of moisture content to cause an electrolyte membrane to be dried, causing a fear of the occurrence of a drop in power output during restart of the fuel cell.
In contrast, if much moisture content remains in the fuel cell, probabilities take place in which the air purge operation is terminated with no adequate removal of moisture content from the fuel cell. Under such a situation, excess moisture content remains in the fuel cell, thereby causing fears in that reaction gases cannot permeate while causing moisture content to freeze under low temperature environments.
As set forth above, if the fuel cell suffers from the shortage of moisture content, the electrolyte membrane is dried with an increase in internal resistance, resulting in a drop in power output.
On the contrary, if the fuel cell has moisture content in excess, catalysts of the electrode are covered with water, causing disturbance in diffusing reaction gases. Therefore, there is a need for air to be supplied at a higher flow rate than that of air to be supplied during normal operation to push out water from the fuel cell or for exhausting hydrogen to the outside of the system for thereby purging water. This results in an extra amount of power consumption and an extra amount of hydrogen being wastefully discharged to the system outside, resulting in a reduction in operating efficiency.
In order to operate the fuel cell at a high efficiency, a need arises in providing a control method of precluding both the shortage of moisture content and the occurrence of excess moisture content.
To this end, Japanese Patent Provisional Publication No. 2002-164069 proposes a fuel cell system in which a wet condition of an electrolyte membrane is discriminated whereupon when the wet condition remains insufficient, a coolant water temperature is lowered to decrease an operating temperature of a fuel cell for thereby increasing a relative humidity to facilitate the wetting of the electrolyte membrane.
With such a fuel cell system, although only the occurrence of the shortage of water is discriminated to decrease the operating temperature of the fuel cell, if the operating temperature of the fuel cell is regulated at an excessively low level, excess moisture content occurs inside the fuel cell. In this respect, no suggestion is made in the above patent literature about a method of controlling moisture content in the fuel cell in just proportion during the occurrence of a drop in the temperature of the fuel cell.
In the related art fuel cell system, further, proposals have heretofore been made to utilize a circulation system wherein unreacted hydrogen (unreacted fuel gas), which is not used for reaction, is circulated through a hydrogen circulation flow passage to a fuel gas supply flow passage for reuse. Another technology is to expel unreacted hydrogen to a system outside through a hydrogen exhaust flow passage.
In any system set forth above, if the fuel cell is caused to operate under a condition where the hydrogen circulation flow passage or the hydrogen exhaust flow passage are closed or nearly closed, nitrogen and steam pearmeate through a positive electrode (oxidant electrode), causing a remarkable drop in hydrogen concentration especially at an area close proximity to a hydrogen outlet portion. This is because resulting nitrogen and steam mearmeating through the electrode disturbs the flow of hydrogen.
If the fuel cell continues to operate under such a condition where hydrogen does not sufficiently diffuse to the hydrogen outlet portion in the unit cell, a drop occurs in the cell voltage not only with a resultant drop in efficiency but also with a resultant deterioration in the electrode at an area where a shortage of hydrogen takes place. Such an issue can be addressed by increasing a rate of supplying hydrogen or by periodically exhausting hydrogen to the system outside. However, blindly exhausting hydrogen to the system outside leads not only to deterioration in efficiency but also to deterioration in safety. Therefore, there is a need for a control method that satisfies requirements both for permitting the hydrogen concentration at the hydrogen outlet portion of the unit cell to be maintained at a level not less than a lower limit and for reducting the flow rate of hydrogen to be exhausted.
To this end, Japanese Patent Provisional Publication No. 2000-243417 proposes a fuel cell system wherein a hydrogen concentration in a hydrogen circulation flow passage is detected by a hydrogen concentration censor whereby when the hydrogen concentration decreases in the hydrogen circulation flow passage, hydrogen is exhausted to the atmosphere.
With such a fuel cell system, there is a need for gas to be sufficiently brought into contact with the hydrogen concentration sensor and the rate of hydrogen to be circulated is ensured to some extent, causing an increase in load of a pump for circulating hydrogen gas.
Moreover, in the non-circulation system, almost no circulation of hydrogen occurs and, hence, it is hard to correctly measure hydrogen concentration. In the non-circulation system, if hydrogen is exhausted to the system outside at increased frequencies for the purpose of correctly measuring the hydrogen concentration, another issue arises where the flow rate of hydrogen to be expelled to the system outside increases.
Besides, as already noted above, in order to precisely control the operating conditions of the fuel cell depending on individual factors, related to individual operating characteristics of the fuel cell, an output voltage of the unit cell forming the fuel cell is measured and depending upon the resulting measured values, the occurrence of a defect in the fuel cell is detected during operation of the fuel cell. Also, it has been a usual practice to inspect electric current-voltage characteristics of the fuel cell.
However, abnormal occurrence factors occurring during operation of the fuel cell include a shortage of oxygen to be supplied, a shortage of hydrogen to be supplied and an increase in internal resistance of the electrolyte membrane, and these defects are resulted in the form of a drop in voltage of the unit cell. As a consequence, merely measuring the voltage of the unit cell makes it hard to specify the abnormal occurrence factors during operation of the fuel cell, causing a difficulty of executing proper operation so as to count the issue depending on a particular factor.
Further, in the event of an increase in irregularities in quality of a processed surface of the unit cell, irregularities in contact resistance of the processed surface of the unit cell increases, causing an increase in irregularities of current density. The unit cell, with such an increase in the processed quality, undergoes an issue where electric current tends to collectively flow through a partial area, resulting in deterioration of the unit cell at increased speeds as compared to another unit cell processed with an appropriate quality. If the stack of unit cells partly contains a defective unit cell with increased irregularities in the processed quality, that unit cell comes to be inoperative in an early stage and even if the other non-defective unit cells operates normally, a battery stack becomes inoperative. If the fuel cell is made of a stack of non-defective unit cells by preliminary rejecting the defective unit cell that has increased irregularities in the processed quality, the battery stack is able to have a prolonged battery life. However, when inspecting the electric current-voltage characteristics on a manufacturing stage, a difficulty is encountered in finding out whether the stacked unit cells partly include the defective unit cell having increased irregularities in the processed quality.
Additionally, another issue arises in a difficulty of specifying the abnormal occurrence factors of the electric power unit, such as the air battery, secondary battery and the capacitors, etc., each including unit cells to generate electric energy, in use or detecting the defective unit cell on the manufacturing stage.
U.S. patent application Publication NO. 2004/95127A1 discloses a current density measuring apparatus arranged to detect current densities of an electrode of a fuel cell. The current density measuring apparatus includes a sensor mounting plate that carries a large number of Hall elements provided at positions corresponding to measuring points on an electrode surface.
With such a structure, the current density measuring apparatus is complex in construction and expensive to manufacture. Another big problem arises in a difficulty with measuring individual operating characteristics related to various parts of the electrolyte membrane or the electrode of the electric power unit in order to specify a particular operating factor for diagnosing individual defects occurring in the electric power unit. Also, if the fuel cell carrying such a current density measuring apparatus is placed in the vicinity of other electric power units, such as an inverter, and an electric motor, the current density measuring apparatus suffers from disturbances of magnetic fields generated by the inverter and the electric motor, resulting in reduction in reliability of measuring the current density. No measure is undertaken in the above related art to count the magnetic disturbances, resulting in a difficulty of increasing reliability of operation.